A geographical map of Africa, showing the ecological break that defines the Saharan area.
The permanent dissolution of clouds allows unhindered light and thermal radiation. The stability of the atmosphere above the desert prevents any convective overturning, thus making rainfall virtually non-existent. As a consequence, the weather tends to be sunny, dry and stable with a minimal risk of rainfall. Subsiding, diverging, dry air masses associated with subtropical high-pressure systems are extremely unfavorable for the development of convectional showers. The subtropical ridge is the predominate factor that explains the hot desert climate (Köppen climate classification BWh) of this vast region. The lowering of air is the strongest and the most effective over the eastern part of the Great Desert, in the Libyan Desert which is the sunniest, the driest and the most nearly rainless place on the planet rivaling the Atacama Desert, lying in Chile and Peru.
The rainfall inhibition and the dissipation of cloud cover are most accentuated over the eastern section of the Sahara rather than the western. The prevailing air mass lying above the Sahara is the continental tropical (cT) air mass which is hot and dry. Hot, dry air masses primarily form over the North-African desert from the heating of the vast continental land area, and it affects the whole desert during most of the year. Because of this extreme heating process, a thermal low is usually noticed near the surface, and is the strongest and the most developed during the summertime. The Sahara High represents the eastern continental extension of the Azores High, centered over the North Atlantic Ocean. The subsidence of the Sahara High nearly reaches the ground during the coolest part of the year while it limits to the upper troposphere during the hottest periods.
The effects of local surface low pressure are extremely limited because upper-level subsidence still continues to block any form of air ascent. Also, to be protected against rain-bearing weather systems by the atmospheric circulation itself, the desert is made even drier by his geographical configuration and location. Indeed, the extreme aridity of the Sahara can't be only explained by the subtropical high pressure. The Atlas Mountains, found in Algeria, Morocco and Tunisia also help to enhance the aridity of the northern part of the desert. These major mountain ranges act as a barrier causing a strong rain shadow effect on the leeward side by dropping much of the humidity brought by atmospheric disturbances along the polar front which affects the surrounding Mediterranean climates.
An oasis in the Ahaggar Mountains. Oases support some life forms in extremely arid deserts.
Condensation occurs on the windward side producing some cloudiness and rainfall while the leeward side stays sunny and dry due to the evaporation process. The leeward side is obviously the desert. This range prevents the unstable weather systems to penetrate further in the interior and to affect these arid regions. That's why there is a sharp decrease in rainfall between the adjacent Mediterranean climates and the desert climate of the Sahara.
The primary source of rain in the Sahara is the equatorial low a continuous belt of low-pressure systems near the equator which bring the brief, short and irregular rainy season to the Sahel and the southern Sahara. The Sahara doesn't lack precipitation because of a lack of moisture, but due to the lack of a precipitation-generating mechanism. Rainfall in this giant desert has to overcome the physical and atmospheric barriers that normally prevent the production of precipitation. The harsh climate of the Sahara is characterized by extremely low, unreliable, highly erratic rainfall; extremely high sunshine duration values; high temperatures year-round; negligible rates of relative humidity, a significant diurnal temperature variation and extremely high levels of potential evaporation which are the highest recorded worldwide.
The sky is usually clear above the desert and the sunshine duration is extremely high everywhere in the Sahara. Most of the desert enjoys more than 3,600 h of bright sunshine annually or over 82% of the time and a wide area in the eastern part experiences in excess of 4,000 h of bright sunshine a year or over 91% of the time, and the highest values are very close to the theoretical maximum value. A value of 4,300 h or 98% of the time would be recorded in Upper Egypt (Aswan, Luxor) and in the Nubian Desert (Wadi Halfa). The annual average direct solar irradiation is around 2,800 kWh/(m2 year) in the Great Desert. The Sahara has a huge potential for solar energy production. The constantly high position of the sun, the extremely low relative humidity, the lack of vegetation and rainfall make the Great Desert the hottest continuously large area worldwide and certainly the hottest place on Earth during summertime in some spots. The average high temperature exceeds 38 °C (100.4 °F) - 40 °C (104 °F) during the hottest month nearly everywhere in the desert except at very high mountainous areas. The highest officially recorded average high temperature was 47 °C (116.6 °F) in a remote desert town in the Algerian Desert called Bou Bernous with an elevation of 378 meters above sea level.
Rocky mountains naturally sculpted by the wind
It's the world's highest recorded average high temperature and only Death Valley, California rivals it. Other hot spots in Algeria such as Adrar, Timimoun, In Salah, Ouallene, Aoulef, Reggane with an elevation between 200 and 400 meters above sea level get slightly lower summer average highs around 46 °C (114.8 °F) during the hottest months of the year. Salah, well known in Algeria for its extreme heat, has an average high temperature of 43.8 °C (110.8 °F), 46.4 °C (115.5 °F), 45.5 (113.9 °F). Furthermore, 41.9 °C (107.4 °F) in June, July, August and September. In fact, there are even hotter spots in the Sahara, but they are located in extremely remote areas, especially in the Azalai, lying in northern Mali. The major part of the desert experiences around 3 – 5 months when the average high strictly exceeds 40 °C (104 °F). The southern central part of the desert experiences up to 6 – 7 months when the average high temperature strictly exceeds 40 °C (104 °F) which shows the constancy and the length of the really hot season in the Sahara. Some examples of this are Bilma, Niger and Faya-Largeau, Chad. The annual average daily temperature exceeds 20 °C (68 °F) everywhere and can approach 30 °C (86 °F) in the hottest regions year-round. However, most of the desert has a value in excess of 25 °C (77 °F). The sand and ground temperatures are even more extreme.
During daytime, the sand temperature is extremely high as it can easily reach 80 °C (176 °F) or more. A sand temperature of 83.5 °C (182.3 °F) has been recorded in Port Sudan. Ground temperatures of 72 °C (161.6 °F) have been recorded in the Adrar of Mauritania and a value of 75 °C (167 °F) has been measured in Borkou, northern Chad. Due to lack of cloud cover and very low humidity, the desert usually features high diurnal temperature variations between days and nights. However, it's a myth that the nights are cold after extremely hot days in the Sahara. The average diurnal temperature range is typically between 13 °C (55.4 °F) and 20 °C (68 °F). The lowest values are found along the coastal regions due to high humidity and are often even lower than 10 °C (50 °F), while the highest values are found in inland desert areas where the humidity is the lowest, mainly in the southern Sahara. Still, it's true that winter nights can be cold as it can drop to the freezing point and even below, especially in high-elevation areas.
(source)